This invention relates to composite electroplated steel strips having improved properties such as weldability and corrosion resistance without painting, and post-painting properties including corrosion resistance and paint adherence, and a method for producing the same.
Zinc deposited steel strips are widely employed as rust preventive steel strips in applications requiring corrosion resistance such as automobiles, electric appliances, and building materials. The pure zinc layer deposited on steel has the sacrificial corrosion prevention effect. That is, since zinc is less noble than the iron substrate, the zinc layer is preferentially corroded rather than pinholes and other plating defects and those portions of the iron substrate exposed during certain working process, thus preventing red rust from generating in the steel substrate. Pure zinc, however, forms upon salt water spraying or in a wet environment electro-conductive corrosion products which rapidly grow. The growth of corrosion products of zinc under a paint coating undesirably causes the paint coating to blister and eventually peel off. These drawbacks are due to the activity of pure zinc.
Other attempts to improve the corrosion resistance of Zn platings include alloying or codepositing zinc with a metal more noble in electrical potential than zinc, for example, Co, Ni, Cr, and Fe in order to suppress the activity of Zn platings. A number of patents and publications describe such attempts as will be explained hereinafter.
(1) Japanese Patent Publication No. 47-16522 discloses to add Co, Mo, W and Fe to a Zn plating bath.
(2) Japanese Patent Publication No. 49-19979 discloses to introduce an oxide of Mo, W or Co and/or Ni, Sn, Pb and Fe into a Zn plating layer.
(3) Japanese Patent Publication No. 56-517 discloses to carry out electroplating in a Zn plating bath having Co, Cr.sup.3+, Cr.sup.6+, In, and Zr added thereto, thereby producing a Zn plating layer having improved corrosion resistance without painting as well as improving its adaptability to chromate treatment.
(4) Japanese Patent Publication No. 58-56039 discloses to carry out electroplating in an acidic Zn plating bath containing a trivalent chromium salt in an amount of at least 3 g/l of Cr.sup.3+, thereby obtaining a Zn-Cr deposit having uniform excellent surface tone and luster and improved corrosion resistance.
The plated steel strips obtained by these methods exhibit improved corrosion resistance without painting over pure zinc layers, but have a problem with respect to corrosion resistance after painting. When these plated steel strips are subjected to a phosphate treatment and then to cationic electrophoretic paint deposition, the resulting paint films tend to blister. Besides, method (3) mentioned above carries out electro-plating in a bath containing Cr.sup.3+ and Cr.sup.6+ for the purpose of improving the adaptability of zinc plated steel to chromate treatment, and thus improving corrosion resistance after chromate treatment. This method does not improve the corrosion resistance of a plating layer itself or the corrosion resistance thereof with a paint film formed thereon by cationic electrophoretic deposition process subsequent to phosphate treatment.